The invention relates to a top-fired primary reformer as well as to a method for catalytic reformation of hydrocarbons with steam under elevated pressure to produce synthesis gas. Synthesis gas of this kind, for example, serves to produce ammonia, hydrogen, and methanol. Accordingly, the primary reformer is so designed and constructed that it counteracts a formation of detrimental nitric oxides in flue gas.
Reactors for catalytic reformation of hydrocarbons with steam have been known for a long time and are known in a plurality of layouts. For large-capacity plants, a design has paved its way in which a top-fired box-type furnace with upright standing reaction tubes and/or split tubes is implemented. The split tubes are arranged in series. The tubes are passed through from top to bottom by process gas which forms the input gas. The input gas is subjected to a so-called splitting process.
The gas outlet temperatures usually range at 850° C. and beyond. In the lower area—inside or outside the furnace—the process gas is collected in so-called outlet collectors. Burners firing vertically downwards are arranged in the “lanes” lying between the tube rows. This area is designated as furnace box. On average, the temperatures in the furnace box range between 1000 and 1250° C. For thermal insulation and for protection from high temperatures prevailing due to heating, the furnace walls are lined with a protective refractory lining.
In its lower area, the furnace chamber in which the firing devices are arranged has a chamber for collection of flue gases as well as a multitude of mainly horizontally arranged bricked tunnels extending in parallel to each other and perpendicular to the vertical tubes. Generated flue gas streams from top to bottom through the furnace and is discharged through those tunnels which have apertures at their sides.
WO2005/018793 A1 describes a typical furnace system and a method for catalytic reformation of hydrocarbons with steam at elevated pressure to obtain synthesis gas. A special configuration of the external walls of the tunnels is applied in order to achieve a better homogenization of the flue gas flow and to obtain a more uniform temperature distribution of the furnace firing. WO2005/018793 A1 describes a typical furnace system and a method for catalytic reformation of hydrocarbons with steam to obtain synthesis gas by supplying oxygen to adapt the stoichiometry and with a special pore burner installed further downstream to avoid formation of soot.
All the reforming systems described hereinabove have in common that a firing device comprised of a multitude of burners arranged between process managing reaction tubes heats the oven chamber with the reforming tubes leading through the furnace chamber. Burners serving for firing the oven chamber are usually supplied with fuel gas and air through separate channels. The supply of fuel gas into the burner chamber is accomplished separately from the supply of air. The penetration of gas feeders into the burner chamber is accomplished through the refractory furnace lining or immediately in front of it. Accordingly, the ratio between fuel gas and air for the burners is controlled by a butterfly flap or a similarly designed facility for the adjustment of the gas flow of the air supply. The burner firing and thus the furnace temperature can be controlled via this facility.
The ratio between oxygen and fuel gas can technically be described by the so-called Lambda (λ) value. On applying a stoichiometrical mol ratio of oxygen versus fuel gas, one obtains a Lambda value of 1.0. On using an oxygen portion which is lower in the stoichiometrical combustion ratio, one obtains a Lambda value which is lower than 1.0. Applying an oxygen portion which is higher in the stoichiometrical combustion ratio, one obtains a Lambda value which is higher than 1.0. Therefore, combustion is optimal if the Lambda value amounts to 1.0. With conventional designs, one obtains Lambda values at the individual burners which fluctuate due to operation and which may have temporarily higher values.
This takes an adverse effect on the combustion process. Its consequence may be a higher total consumption of fuel gas relative to the turnover of the reforming process. With a change of the fuel material, the supply of air can hardly be adjusted to the modified stoichiometry. Consequently, it may temporarily entail an unintentional increase in the flame temperature and, as a result of an increased inflow of air, it may involve an intensified formation of nitric oxides of the NOx type. As pollutants in the atmosphere, nitric oxides contribute to acid rain.
It is also known that the nitric oxide contents NOx of a waste gas decreases substantially when applying a more favorable Lambda value at the burner brick. And it is well known that the nitric oxide contents NOx of a waste gas decreases substantially when adjusting and setting a lower flame temperature. This may be gathered from the relevant and well known literature. To give an example, reference is made here to the teaching “The John Zink Combustion Handbook”, C. E. Baukel Jr., CRC-Press, London New York, 2001. Therefore, an optimized adjustment and setting of the air vs. fuel gas ratio at the burners and an optimal control of combustion with regard to the adjustment and setting of an optimal Lambda value are of essential importance in the reduction of nitric oxides in synthesis gas production.
With certain operating conditions, such as in partial load mode, prior art designs moreover pose a problem in that the volume of evolving flue gas must be raised by increasing the air surplus in order to adapt heat transfer in the waste heat section downstream of the reformer to the operational requirements of the overall plant. An increased air surplus takes a negative effect on the formation of nitric oxides in flue gas.
WO2008/131832 A1 describes a reactor for catalytic reformation of hydrocarbons with steam at elevated pressure, said reactor comprising a reaction chamber and a firing chamber, said reaction chamber comprised of a plurality of vertical tubes arranged in rows and suitable for being filled with a catalyst, and having facilities for feeding of hydrocarbons and steam to be reformed to the reaction chamber, and furthermore comprising facilities for discharge of reformed synthesis gas from the reaction chamber, and furthermore comprising a plurality of firing facilities in the upper area of the firing chamber, said firing facilities being able to generate mainly downwardly directed flames that are suitable to heat the afore-mentioned reaction tubes, with the tube feeding air to the burner being equipped with a facility for adjusting and setting the air flow, and there being a secondary air feeder mounted additionally to this tube and branching-off from it and configured in various layouts and having an independently controllable facility for adjusting and setting the air flow and also feeding air to the firing facility so that a more favorable ratio of fuel gas versus air results at the burners so as to be able to achieve a waste gas poor in nitric oxides.
This configuration bears a disadvantage in that the burners proper have to be of a very sophisticated configuration in order to equip them with the a.m. secondary inlet ducts for air.